Conventionally, in order to share information with plural apparatuses, a plurality of data transmission apparatuses each having a transmission unit for transmitting data and a reception unit for receiving data are provided, and the respective data transmission apparatuses are connected in a ring shape to constitute a network.
FIG. 11 is a block diagram illustrating the construction of a conventional ring-shaped network.
With reference to FIG. 11, the conventional ring-shaped network is provided with a plurality of data transmission apparatuses, and each data transmission apparatus is connected to a next-stage data transmission apparatus through a transmission path, and a final-stage data transmission apparatus is connected to an initial-stage data transmission apparatus.
A data transmission apparatus 1010 is provided with a data transmission unit 1011 for transmitting data; an E/O 1014 for converting an electric signal into an optical signal; an optical fiber cable 1016 for transmitting the optical signal; an O/E 1015 for converting an optical signal into an electric signal; a data reception unit 1012 for receiving data; and a clock reproduction unit 1013 for establishing clock synchronization on the basis of the received data. Likewise, a data transmission apparatus 1020 is provided with a data transmission unit 1021, an E/O 1024, an optical fiber cable 1026, an O/E 1025, a data reception unit 1022, and a clock reproduction unit 1023. Further, a data transmission apparatus 1030 is provided with a data transmission unit 1031, an E/O 1034, an optical fiber cable 1036, an O/E 1035, a data reception unit 1032, and a clock reproduction unit 1033.
The initial-stage data transmission apparatus 1010 is connected to the next-stage data transmission apparatus 1020 through the transmission path 1016, the data transmission apparatus 1020 is connected to the data transmission apparatus 1030 through the transmission path 1026, and the final-stage data transmission apparatus 1030 is connected to the initial-stage data transmission apparatus 1010 through the transmission path 1036, thereby constituting a ring-shaped network.
Further, the data transmission apparatus 1010 is a master which transmits data with a clock of its own, and the data transmission apparatuses 1020 and 1030 are slaves which transmit data after clock synchronization is established on the basis of the received data.
Next, the operation of initializing the ring-shaped network so constructed will be described.
Initially, the data transmission apparatus 1010 that functions as a master transmits data to the next-stage data transmission apparatus 1020 with a clock of its own. In the data transmission apparatus 1020, the clock reproduction unit 1023 performs clock reproduction from the received data to establish synchronization, and thereafter, outputs the data to the next-stage data transmission apparatus 1030. In repetition of this operation, when clock synchronization is established by the clock reproduction unit 1013 of the master data transmission apparatus 1010, the ring-network is initialized. When initialization of the ring-shaped network is completed, the respective data transmission apparatuses perform data communication through the network.
There are cases where devices are connected to the respective data transmission apparatuses although they are not shown in the figure. These connected devices perform various kinds of processing on the basis of the data received by the data reception units 1012, 1022, and 1032, and output the results to the data transmission apparatuses 1011, 1021, and 1031, respectively.
In order to initialize the ring-shaped network in which the data transmission apparatuses having the connected devices are connected, initially, synchronization of the data transmission apparatus 1020 is established, and synchronization of the device connected to the apparatus 1020 is established with the synchronization clock. Thereafter, synchronization of the data transmission apparatus 1010 is established, and synchronization of the device connected to the apparatus 1010 is established with the synchronization clock.
However, in the conventional ring-shaped network, after clock synchronization of the previous-stage data transmission apparatus is established, the previous-stage apparatus outputs a signal to the next-stage data transmission apparatus, and the next-stage data transmission apparatus receives the signal to establish clock synchronization. Therefore, initialization takes much time.
Further, when devices are connected to the data transmission apparatuses, synchronization must be established in the respective connected devices, and therefore, initialization takes more time.
Moreover, although FIG. 11 shows an example of a ring-shaped network in which three-stages of data transmission apparatuses are connected, more data transmission apparatuses are connected in an actual ring-shaped network. Therefore, the time required for initialization increases according to the number of data transmission apparatuses connected.
The present invention is made to solve the above-described problems and has for its object to provide a ring-shaped network and a data transmission apparatus, which can perform initialization at high speed even when performing multi-valued transmission.